The moon is more than just an illuminating fixture on a dark night sky. Together with the sun, it is responsible for the rise and fall of sea levels. Some places even experience two low and two high tides a day.
Many factors contribute to tides: the shape of the beach, the size of the coastline, the angle of the seabed that leads up to the shore, prevailing winds and ocean currents, among others.
As the moon wanes and waxes from its different phases, it either creates larger tides known as spring tides, or smaller tides known as neap tides.
Turns out, those aren't the only phenomena affected by the phases of the moon. A new study conducted by the University of Washington suggests that when the moon is high up in the sky, it creates bulges in the atmosphere that change the amount of rainfall on Earth.
How Lunar Forces Affect the Amount of Rainfall
Air pressure changes associated with moon phases were first recorded in 1847. In 1932, the changes were linked to temperature.
Tsubasa Kohyama, an atmospheric sciences doctoral student, observed a slight fluctuation in the Earth's air pressure while studying atmospheric waves. Eventually, this observation prompted Kohyama and atmospheric sciences professor John Wallace to track down the occurrence. This lasted for two years.
What strengthened Kohyama and Wallace's study were findings from a recent UW study, which used a global grid of data to confirm that air pressure on the surface of the Earth indeed varies with the moon's phases.
"When the moon is overhead or underfoot, the air pressure is higher," said Kohyama.
Kohyama and Wallace's new paper is the first to describe how the moon's gravitational tug on Earth dampers rainfall.
How does that happen? Scientists explained that when the moon is overhead, its pull causes the atmosphere of the Earth to bulge toward it. The weight or the pressure of the atmosphere on this side goes up.
The Earth's atmosphere can be likened to a part of a very large balloon with strong internal gravitational force, while the moon's gravity is the pressure that comes from a small vacuum cleaner. Although the pressure is different compared to what happens to these objects, the moon's force is trying to suck the Earth's atmosphere closer to it, while simultaneously affecting the volume of the air inside.
According to Kohyama, higher atmospheric air pressure increases the temperature of air particles on the surface. Because warmer air can hold more moisture, particles of air are now farther from their moisture capacity.
"It's like the container becomes larger at higher pressure," he said.
This moisture capacity or humidity affects the amount of rain. Kohyama said lower humidity is less likely to result to precipitation.
Studying Climate Models
Kohyama and Wallace examined data from NASA and the Tropical Rainfall Measuring Mission satellite of the Japan Aerospace Exploration Agency. The 1998 to 2012 records revealed that rain is lighter when the moon is high.
However, the change is only about one percent of total rainfall variation. It's not enough to affect aspects of weather, or to make people notice the difference.
"No one should carry an umbrella just because the moon is rising," said Kohyama.
Still, their findings can be applied in testing climate models to check if their physics is enough to replicate how the moon's gravitational pull influences rainfall.
Kohyama and Wallace's study is featured in the journal Geophysical Research Letters.